code: def test_multi_thread(self):
      mel_spectrogram = AudioLoader.load_mel_spectrogram(
          self.sample_wav_file, n_mels=128
      )
      n = 3
      augs = [nas.FrequencyMaskingAug(), nas.TimeMaskingAug()]
  
      for num_thread in [1, 3]:
          for aug in augs:
              augmented_data = aug.augment(
                  mel_spectrogram, n=n, num_thread=num_thread
              )
              self.assertEqual(len(augmented_data), n)
  
file_path: nlpaug/test/augmenter/spectrogram/test_spectrogram.py
imports: import unittest
  import os
  from dotenv import load_dotenv
  from nlpaug.util import AudioLoader
  import nlpaug.augmenter.spectrogram
label_desc: Use image node_id 2 for calling the TestFrequencyMasking obj's underlying member method code with example usage: obj.test_multi_thread() without return types